Insertion block for the formation of a hook field on an injection-moulded object, and a moulded object comprising a hook field of this type

ABSTRACT

Object moulded in thermoplastic material in any shape having an outer surface from which protrudes at least one hook in one piece with the moulded object, in particular a hook field, the moulded object and the hook(s) having been formed by injection moulding, each hook comprising a base part and a head or hooking part protruding from the base part and being delimited by first and second lateral surfaces each extending on the outer surface of the moulded object, being separated from each other by an intermediate surface forming the edge of the hook, the curve sections, defined by the intersections of at least one of the first and/or second lateral surfaces with planes which are in parallel with the base plane from which the hook protrudes and which are at given distances h from this base plane, have a curvature which varies as a function of the distance h, the curvature of the curve section for h=0 (the base plane and said parallel plane merging) being greater than the curvature of at least one curve section for a distance h substantially corresponding to a level of the head of the hook.

TECHNICAL FIELD

The present invention relates to a moulding insertion block intended tobe placed in a mould for forming a moulded object, the block beingintended to form a hook field in one piece with the moulded object. Thepresent invention also relates to a moulded object comprising a hookfield produced in one piece with the moulded object.

BACKGROUND ART

When an object of any shape is moulded, in particular by injection, anda hook field is desired to be provided thereon which will enable theobject to be attached, for example via loops of a woven material orother hooks which co-operate with the hook field, an insertion block ismade beforehand which is intended to be inserted in the mould forproducing the moulded object and comprises in general a field ofcavities issuing at its upper surface and having a shape complementaryto that of the hooks of the hook field to be formed. This insertionblock is then disposed in the mould in which the moulded object will bemoulded, the hooks thus being formed at the same time as the mouldedobject and in one piece therewith.

The insertion blocks known nowadays are formed of plates stacked oneagainst the other, and on the upper edge or field of some of theseplates, cut-outs having a shape complementary to that of the hooks areformed, by machining e.g., by spark-machining, laser etching, chemicalcutting or the like. The stacked plates are generally formed of twotypes of plates, i.e., plates whose fields have cut-outs and plateswithout cut-outs—these latter plates being disposed between two plateswith cut-outs to thereby form the cavities, each cavity being formed ofone cut-out and the lateral walls of the adjoining plates withoutcut-outs. However, plates can also be stacked all having cut-outs ontheir field, by offsetting the cut-outs formed in two successive platessuch that the part of one plate between two of its successive cut-outsforms a lateral wall of the cavities of the adjoining plates.

Once the plates are stacked one on top of the other, these are pressedtogether by jaw-type systems and/or are welded at the periphery thereofto keep them together, wherein a combination of these methods is alsopossible.

These insertion blocks of the prior art have many drawbacks:

-   -   when they are placed in the mould for forming the moulded object        on which they are intended to permit the addition of the hook        field, the plates which are clamped together or are welded        together at their ends might slightly separate from each other        under the effect of the injection pressure of the thermoplastic        material allowing the passage of the thermoplastic material        between two successive plates. Upon exiting the mould, the hook        field formed on the moulded object thus has a burr formed by a        sort of wall, often with a greater height than the hooks and        extending over substantially the entire width of the hook field,        wherein this burr might make the hook field unusable or at least        non-compliant for fixation to, for example, another hook field        or to loops of a woven material. This also makes the moulded        object as a whole unusable, which object must be scrapped.        Furthermore, the insertion block would then have to be repaired,        i.e., the process of producing the moulded object would have to        be stopped and it would have to be ensured that the plates        forming the insertion block are better clamped together. This,        of course, results in costs in terms of time, personnel and        productivity.

On the other hand, these insert blocks of the prior art make itdifficult, owing to the need to clamp and keep the plates against eachother, to produce inserts with a small thickness, e.g., less than 10 mm.

Furthermore, the current inserts require a device to keep the platesclamped together which means that the introduction of the insert and ofthis clamping/holding device into the mould requires a lot of space,resulting in particular in a used surface area which is much greaterthan the surface area taken up by the hook field formed by the insert.

DISCLOSURE OF THE INVENTION

The invention aims to overcome the drawbacks of the prior art byproposing an insertion block of the type mentioned above which enablesthe appearance of burrs in the hook field after injection moulding to becompletely or at least substantially avoided. On the other hand, it isalso possible to obtain inserts which are very thin, having a surfacearea substantially equal to that of the hook zone on the moulded object.

By attaching the plates to each other by this process, which is apiece-joining process for forming a solid piece, which does not requirea liquid interface as in brazing, and which does not produce a porousconnection by melting and re-solidification as in conventional fusionwelding. An insertion block is obtained which is particularly resistantto the injection pressure and since there is no longer an interfacebetween the plates, it is no longer possible for the thermoplasticmaterial to infiltrate therein to create a burr since the plates can nolonger separate from each other since they are diffusion bondedtogether.

By thereby providing an aeration channel, the air pushed to the bottomof the cavity by the thermoplastic material injected into the cavity isable to escape via the bottom thereof. Although the clamping force isthe same, the thermoplastic material is less likely to infiltratebetween the plates and to form burrs.

Preferably, the at least one channel is formed so as to communicate withthe part intended to form the head of the hooks of the cavity.

In accordance with one preferred embodiment of the invention, theaeration channel(s) has/have a stepped cross-section formed by a firstpart having a small cross-section and a second part having a largercross-section than the small cross-section, wherein the smallcross-section is intended to effect the communication with the head partof the cavity having a shape complementary to the hooks such that aircan pass into this cross-section but not thermoplastic material, and thelarge cross-section permits quick venting of the air which passes fromthe small cross-section into the large cross-section, itself in contactwith the outside.

In accordance with another preferred embodiment of the invention, thecross-section of the channel(s) is flared, starting with a smalldimension through which air can pass but not injected thermoplasticmaterial and finishing with a large dimension permitting quick ventingof the air.

In accordance with a preferred embodiment, the at least one auxiliarycut-out is formed in the at least one first plate.

Preferably, the second plates are plates without cut-outs.

In accordance with one possible embodiment, the second plates have attheir edge at least one second plate cut-out and when the second platesare disposed against the at least one first plate, the second platecavity is offset with respect to the cavity of the first plate such thatthe first plate forms a wall for the second plate cavity and the secondplates each form a wall for the cavity of the first plate.

Preferably, the diameter of the equivalent cross-section of the openingfor communication between the cavity and the aeration channel is lessthan 50 micrometers and is preferably between 10 micrometers and 40micrometers.

The diffusion bonding of at least two plates consists of firmlyassembling the plates against each other by keeping them pressedtogether by a connecting system on all of their surfaces. The mould isthen heated, preferably under vacuum, to a temperature conventionallyequal to about 0.5 to 0.8 times the melting temperature of the materialof the plates. Maintaining the pressure thus results in a diffusion bondbetween the plates. The term “diffusion bonding” refers to the fact thatunder high pressure and with a temperature close to the meltingtemperature, the atoms diffuse between the plates and permit theirrespective bonding. This technique, per se, is well known and isdescribed for example in U.S. Patent Publication No. 2005/0109821.

The present invention also relates to a object moulded in thermoplasticmaterial in any shape having an outer surface forming a base plane fromwhich protrudes at least one hook in one piece with the moulded object.The moulded object and the at least one hook having been formed byinjection moulding, the at least one hook comprising a base part and ahead or hooking part protruding from the base part, and being delimitedby first and second lateral surfaces each extending on the outer surfaceof the moulded object being separated from each other by an intermediatesurface forming the edge of the at least one hook, characterised in thatthe curve sections, defined by the intersections of the first and/orsecond lateral surfaces with the planes which are in parallel with thebase plane and which are at given distances h from this base plane,having respective curvatures which vary as a function of the distance h,the curvature of the curve section for h=0 (the base plane and saidparallel plane merging) being greater than the curvature of at least onecurve section for a distance h substantially corresponding to a level ofthe head of the at least one hook.

In fact, upon fixation of the plates by diffusion bonding a highpressure is applied onto the stacked plates and this results in adeformation of the walls of the cavities formed in these plates. Thisresults in the lateral surfaces having a concave shape, the concavitybeing more pronounced at the level of the base and decreasing towardsthe head of the hook (or towards the bottom of the cavity). Inparticular, at the level of the head, this concavity is substantiallyzero. In contrast, it is often the corners of the cross-section whichbecome rounded based on the material pressure, viscosity conditions etc.

Preferably, the curve sections of the two first and second lateralsurfaces have curvatures such that their concavities are opposed.

Preferably, at least at the level of a so-called transition part of thehead, the transition between at least one of the two lateral surfacesand the intermediate surface is smooth, i.e., with no angular parts orsharp edges, in particular it is curved.

Preferably, the intermediate surface is substantially perpendicular tothe two first and second lateral surfaces.

In fact, in accordance with the use of the novel insertion block inaccordance with the invention, in contrast to the prior art, when thethermoplastic material enters the cavity in the form of a rod, there areno—or there are less—escape routes for the air via the lateral surfaces(and then via the interstices between the plates as in the prior art) atthe level of the base of the hook such that the flow of thermoplasticmaterial is substantially homogenous at the level of the base of therod, or at least more homogenous than in the case of the systems of theprior art. In contrast, at the level of the head, the thermoplasticmaterial will likely flow differently and in particular taking intoaccount the different ventilation of the cavity. In accordance with thesize and position of the aeration channel, air might remain trapped atthe bottom of the cavity, thereby preventing the thermoplastic materialfrom reaching the corners, such that the shape of the heads of the hooksafter extraction thereof from the cavities does not include angularparts, like in the prior art, but in contrast includes curved parts.

In accordance with one preferred embodiment, the thickness (distancebetween the first and second lateral surfaces) of the transverse section(cross-section in a plane parallel to the base plane) decreases from thebase of the hook towards the head.

In accordance with one preferred embodiment, a first cross-section atthe level of the head of the hook has a first cross-section having atleast one rounded corner having a first radius of curvature, and asecond cross-section at the level of the head of the hook has a secondcross-section having at least one rounded corner having a second radiusof curvature, wherein the second cross-section is closer to the distalend of the head and the second radius is greater than the first radius.

In accordance with a preferred embodiment, a cross-sectional development(transversally to the two lateral surfaces and to the intermediatesurface) smaller than the cross-section of the hook head protrudes fromthe distal end of the head of the hook. This is particularly the casewhen the thermoplastic material used is extremely fluid or when theopening between the moulding cavity and the aeration channels is largesuch that the thermoplastic material enters via this opening to protrudeinto the aeration channels.

In accordance with one preferred embodiment, at least one part of thedevelopment forms the so-called transition part of the head whilst theremainder of the hook in cross-section has the shape of a quadrilateralsuch as a square, rectangle, rhombus or the like.

In accordance with one preferred embodiment, the transition between thedevelopment and the head of the hook forms a shoulder or corner.

In the present invention, the term “smooth transition”, i.e., withoutangular parts or sharp edges, in particular in a curved manner, refersto the fact that the curve delimiting the cross-section (i.e.,transversely to the lateral surfaces and transversely to theintermediate surface) of the hook at the level of the transition, whenseen under a microscope at a magnification of at least 150, is a smoothcurve, in particular one whose first derivative is continuous.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the invention, given solely by way of example, willnow be described with reference to the drawings, in which:

FIG. 1 is a side view of a plate of an insertion block;

FIG. 2 is a perspective view of an insertion block in accordance withthe invention formed of a stack of plates shown in FIG. 1, whereinplates without cavities have been interposed between said plates;

FIGS. 3A and 3B are top views of one part of the stack of plates of FIG.2, before and after diffusion bonding respectively;

FIG. 4 shows a moulded object having a hook field, the moulded objectand the hooks being in one piece and obtained by injection moulding in amould in which the insertion block has been disposed beforehand to formthe hook field;

FIG. 5 shows a perspective view of a hook of a hook field obtained usinga moulding insert in accordance with one embodiment of the invention;

FIG. 6 is a cross-sectional view taken along line A-A of FIG. 5;

FIG. 7 is a cross-sectional view taken along line B-B of FIG. 5;

FIG. 8 is a cross-sectional view taken along line C-C of FIG. 5;

FIG. 9 is a cross-sectional view taken along line D-D of FIG. 5;

FIG. 10 is a cross-sectional view in the base plane P0 of the hook;

FIG. 11 shows a perspective view of a hook of a hook field obtainedusing a moulding insert in accordance with another embodiment of theinvention; and

FIG. 12 shows a perspective view of a hook of a hook field obtainedaccording to yet another embodiment of the invention.

PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION

The insertion block 1 shown in FIGS. 2 and 3B is formed of analternating stack of plates 2 having cut-outs 4 of a shape complementaryto the hooks, and plates 3 not having cut-outs of a shape complementaryto the shape of the hooks. The plate 2 shown in FIG. 1 is a plate havingtwo lateral faces 5 and an upper edge or field 7 on which cut-outs 4 ofa shape complementary to the hooks intended to be formed are formed byspark-machining. When this plate 2 is sandwiched between two plates 3without cavities in the form of hooks, each cut-out 4 forms, with thewalls of the adjoining plates 3, a cavity closed on the sides by twolateral, planar walls formed by the two plates 3 adjoining the plate 2and this cavity is open at the top by an upper opening forming the baseof a hook. Also formed by spark-machining, in the two lateral surfaces 5of the plate with hooks, are two grooves 8 which will intersect theparts forming the head of the cavity having a shape complementary tothat of a hook. Each groove is stepped, having a first part with a smallcross-sectional dimension such that only air can pass therethrough andnot thermoplastic material, and a second part with a largercross-sectional dimension which enables the air to be vented morequickly. These stepped channels could also be formed not in the platewith hooks but in the intermediate plates without hook-shaped cut-outs,or simultaneously in the intermediate plates and in the plate withhooks.

Once the plates with cut-outs and the intermediate plates have beenformed, they are firmly pressed together and diffusion bonding isperformed to obtain a final block in which all of the elements areconnected. A thermal processing oven can be used for example, fittedwith a cylinder of a Demag press which holds the plates together bymeans of pressure during the cycle in which the atoms diffuse betweenthe plates (under vacuum or in the presence of an inert gas) under thecombined effect of temperature and pressure, the diffusion processpermitting a connection between each adjoining plate at the atomic levelto obtain in the end a block in which all of the elements are connected.At the end of this process, the plates have undergone a compression ofbetween 0.5 and 8%, preferably between 1 and 4%, in particular about 2%and like the empty parts (the cavities were not affected by thiscompression) the walls 30 of the cavities 4 in the plate 2 haveundergone a deformation which provides them with a camber which evensout from the base or opening towards the bottom or head of the cavity.In practice, this deformation is about 2% with respect to the initialthickness of the intermediate plate 3, but this deformation can varybetween 0.5 and 8%, preferably between 1% and 4%. The camber of one ofthe lateral walls has its concavity directed in a direction opposite tothe concavity of the camber of the other lateral wall.

The insertion block of FIG. 2 can thus be disposed in any type of mouldfor forming an injection-moulded object. The object shown in FIG. 4 canbe made for example. A hook field corresponding to the hooks formed bythe cavities of the block 1 protrudes from the moulded object and canthus permit fixation thereof. The surface over which the field extendsis substantially smaller than the total surface of the moulded object,but it is also foreseeable to provide a moulded object where themajority of the surface is covered by the hook field. Upon forming themoulded object, thermoplastic material is injected into the mould inwhich the insertion block has been inserted beforehand substantially atthe location where the hook field which will form one piece with themoulded object is intended to be formed. This thermoplastic materialwill enter the cavities of the insertion block and, owing to thepresence of the aeration channels, will then reach the bottom of thehooks, pushing out the air into the channels.

FIGS. 5 and 6 show hooks as obtained in accordance with the invention.Each hook 10 is delimited by two lateral, substantially planar andparallel surfaces 11 and 12 and by an intermediate surface 13 adjoiningthe two surfaces 11 and 12. The intermediate surface 13 is delimited bythe upper edges of the two planar surfaces, from the bottom of the hookon the right to the bottom of the hook on the left, passing through thetip thereof. The intermediate surface 13 forms the upper edge of thehook. These three surfaces thus delimit the hook which comprises a rodor base part 14 and a head part 15 protruding from the rod part. At thelevel of the lower part of the base or rod, the transition between eachlateral surface and the intermediate surface is formed by a line suchthat the surfaces form an angle therebetween, in particular a rightangle, whilst at the level of a part of the head, for example in theupper part, the transition between the lateral surfaces and theintermediate surface is smooth, i.e., curved. Thus, the shape of thecross-section of the hook at A-A of the head is itself formed, as shownin FIG. 6, of four straight lines 16 connected together by curved partswith large curvatures 17. Based on the location where the cross-sectionis taken, a cross-section which has all of its corners rounded, or onlyone or two or three of its corners, is possible.

The cross-section B-B of the hook of FIG. 5 and shown in FIG. 7 has thesame type of shape as that of the cross-section A-A of FIG. 5, but therounded corners 17′ have a greater radius of curvature than that of therounded corners 17 of FIG. 5.

The cross-section C-C at the level of the base, shown in FIG. 8, hassharp-edged, i.e., non-rounded, corners, forming a sort of turningpoint. However, the two curve sections 20 and 21 which correspond to theintersection between the respective lateral surfaces 11 and 12 and theplane parallel to the base plane P0 and to the distance hc from thisplane are curved having their concavity towards the exterior of the hookand opposed to each other.

Similarly, the cross-section at the level of the base plane, shown inFIG. 10, has sharp-edged, i.e., non-rounded, corners forming a sort ofturning point. However, the two curve sections 22 and 23 whichcorrespond to the intersection between the respective lateral surfaces11 and 12 and the base plane P0 are curved, having their concavitytowards the exterior of the hook and opposed to each other. Thecurvature or concavity of the section 22 is greater than that of thesection 20 and that of the section 23 is greater than that of thesection 21. Similarly, at the level of the head, the sections 24 and 25corresponding to the cross-section along line D-D are substantiallystraight, their concavity thus being zero and therefore less that thatof the sections 20 and 21.

The thickness e0 at the level of the base plane is less than thethickness at the level of the plane C-C which itself is less that thethickness eDD at the level hd of the plane D-D.

In the hook field of the moulded object of FIG. 4, the hooks arearranged in rows and columns, which are in particular perpendicular toeach other.

The thermoplastic moulding material can be selected from polyolefins,polyamides, TPEs, etc.

In the case of FIG. 5, the cavities of the corresponding insertion blockhave an opening towards the aeration channel which has a smallcross-section, and in particular is only present on one side of thehead. This results in some degree of braking for the material which isnot able to perfectly fill the corners of the cavities, and thus inrounded shapes for the head at the corners of the cavity.

In contrast, as shown in FIG. 11, in the case where the opening has alarger cross-section and in particular extends over virtually all of thebottom of the head, the corners are well formed and the hook does nothave a rounded shape, or only has a slight rounded shape.

In the case where the opening between the moulding cavity and theaeration channels is very large and where furthermore the viscosity ofthe material permits an extremely fluid flow, the thermoplastic materialis able to surge into this opening and protrude into the channels. Insuch a case, and as shown in FIG. 12, the hook will comprise at thelevel of its head a development 40 created by this infiltration of thethermoplastic material in the passage opening towards the aerationchannels of the moulding cavity. Often in this case, the transitionbetween the lateral surfaces and the intermediate surface will be formedwith a sharp edge or corner over the entire extent from left to right ofthe hook of the intermediate surface. The cross-section without a sharpedge or corner will be formed in particular at the level of thedevelopment.

To show the gradient of curvature of the lateral surface of the hook(i.e., that at the level of the head the curvature is virtually zero orat least less than the curvature at the level of the base of the hook,the following test can be used:

The hook field of the moulded object must be inclined by 15° withrespect to the horizontal, the inclination being effected around an axisin parallel with the intersection of the transverse plane and of thebase plane of a line of hooks.

The hooks must be observed using a microscope having a magnification ofat least 150, for example the variable pressure, Hitachi S 3200NScanning Electron Microscope, high vacuum observations and secondaryelectrons after Au/Pd metallization.

In order to be able to observe a hook in the middle of the hook field,the hooks surrounding it can be cut using a razor blade.

Cross-sections which are substantially rectangular are shown. However,they can also be simply substantially quadrilateral by cutting with theblades not at 90° with respect to the plane of the plates but sloping.

The invention claimed is:
 1. An injection moulded thermoplastic object,the moulded object comprising: a moulded base having a top that forms abase plane; at least one hook protruding from the base plane and mouldedin one piece with the base, the at least one hook comprising a base partand a head part protruding from the base part, the at least one hookbeing delimited by first and second lateral surfaces each extending fromthe base plane and being separated from each other by an intermediatesurface forming an edge of the at least one hook, the at least one hookhas curve sections defined by intersections of the first and secondlateral surfaces with planes which are parallel with the base plane andwhich are at given distances h from the base plane, the curve sectionshave respective curvatures which vary as a function of the distance h,the curvature of the curve section for h=0 being greater than thecurvature of a curve section for a distance h substantiallycorresponding to a level of the head part of the at least one hook, andwherein the minimum thickness of the base part proximate the base planeis smaller than the minimum thickness of the at least one hook at aplane parallel to the base plane that is substantially at the level ofthe head part of the at least one hook, wherein each curved section isconcave and curves inward toward the interior of the at least one hookso that the curve sections of the first and second lateral surfaces haveopposed concavities.
 2. The injection moulded thermoplastic object as inclaim 1, wherein at least at the level of a transition part of the headpart, the transition between the first and second lateral surfaces andthe intermediate surface is smooth, i.e., with no angular parts or sharpedges, in particular curved.
 3. The injection moulded thermoplasticobject as in claim 1, wherein the intermediate surface is substantiallyperpendicular to the first and second lateral surfaces.
 4. The injectionmoulded thermoplastic object as in claim 1, wherein a firstcross-section at the level of the head part has at least one roundedcorner having a first radius of curvature, and a second cross-section atthe level of the head part has at least one rounded corner having asecond radius of curvature, wherein the second cross-section is closerto the distal end of the head part and the second radius is greater thanthe first radius.
 5. The injection moulded thermoplastic object as inclaim 1, wherein a cross-sectional development is smaller than thecross-section of the head part and protrudes from the distal end of thehead part of the at least one hook.
 6. The injection mouldedthermoplastic object as in claim 1, wherein at least one part of thecross-sectional development forms a transition part having no angularparts or sharp edges in cross section, whilst the remainder of the atleast one hook in cross-section has the shape of a four-corneredquadrilateral such as a square, rectangle, rhombus or the like.
 7. Theinjection moulded thermoplastic object as in claim 1, wherein thetransition between the cross-sectional development and the head part ofthe at least one hook forms a shoulder or corner.
 8. The injectionmoulded thermoplastic object as in claim 1, wherein the minimumthickness of the at least one hook increases from the base plane towardsthe head part.
 9. The injection moulded thermoplastic object as in claim1, wherein the curved sections have their concavities towards theinterior of the at least one hook.
 10. The injection mouldedthermoplastic object as in claim 1, wherein each lateral surface has aconcave shape with its concavity towards the interior of the at leastone hook, the lateral surface being more concave proximate the baseplane and being less concave towards the head part of the at least onehook.
 11. An injection moulded thermoplastic object, the moulded objectcomprising: a base having a top forming a base plane; at least one hookin one piece with the moulded object, the moulded object and the atleast one hook having been formed by injection moulding, the at leastone hook protruding from the base plane and comprising a base part and ahead part protruding from the base part, the at least one hook beingdelimited by first and second lateral surfaces each extending from thebase plane and being separated from each other by an intermediatesurface forming an edge of the at least one hook, curve sections aredefined by intersections of the first and/or second lateral surfaceswith planes which are in parallel with the base plane and which are atgiven distances h from this base plane, the curve sections havingrespective curvatures which vary as a function of the distance h, thecurvature of the curve section for h=0 being greater than the curvatureof at least one curve section for a distance h substantiallycorresponding to a level of the head of the at least one hook, andwherein the curved sections are concave and curve inward towards theinterior of the at least one hook so that the curve sections of thefirst and second lateral surfaces have opposed concavities.
 12. Themoulded object as in claim 11, wherein at least at the level of atransition part of the head part, the transition between at least one ofthe two lateral surfaces and the intermediate surface is smooth anddevoid of sharp edges.
 13. The moulded object as in claim 11, whereinthe intermediate surface is substantially perpendicular to the first andsecond lateral surfaces.
 14. The moulded object as in claim 11, whereina first cross-section at the level of the head part of the at least onehook has at least one rounded corner having a first radius of curvature,and a second cross-section at the level of the head part of the at leastone hook has at least one rounded corner having a second radius ofcurvature, wherein the second cross-section is closer to the distal endof the head part and the second radius is greater than the first radius.15. The moulded object as in claim 11, wherein a cross-sectionaldevelopment is smaller than the cross-section of the head part andprotrudes from the distal end of the head of the at least one hook. 16.The moulded object as in claim 15, wherein at least one part of thedevelopment forms a transition part having non angular parts or sharpedges in cross-section, whilst the remainder of the at least one hook incross-section has the shape of a four-cornered quadrilateral.
 17. Themoulded object as claimed in claim 15, wherein the transition betweenthe development and the head part of the at least one hook forms ashoulder or corner.
 18. The moulded object as in claim 11, wherein theminimum thickness of the at least one hook at the base plane is smallerthan the minimum thickness thereof in a plane parallel to the base planesubstantially at the level of the head part of the at least one hook.19. The moulded object as in claim 11, wherein the minimum thickness ofthe at least one hook increases from the base part toward the head part.20. The moulded object as in claim 11, wherein each lateral surface hasa concave shape with its concavity towards the interior of the at leastone hook, the concavity being more pronounced proximate the base anddecreasing towards the head part of the at least one hook.